The clearest explanation I found of the NAZA calibration process was in "How to Fit Gimbal Tilt Control Lever to Phantom Transmitter Controller TX", a video by Marionville Hobbies (UK) at https://www.youtube.com/watch?v=4f33X7N7MiE
I used the procedure in the video to install a Walkera G-2D on my FC40 and calibrate the tilt lever. Prior to the installation, I was trying to imagine how the control lever's tiny arc of adjustment would translate to the gimbal's full range of motion. Of course, once you understand how NAZA calibration works, the answer is obvious. But for NAZA noobies who are trying to learn what to anticipate, I'd like to offer a few pointers.
1) Prior to calibration in NAZA, my gimbal pointed straight up and did not respond to the lever at all, as described in many forums. You MUST calibrate the gimbal in NAZA before it will function.
2) After you have completed the calibration procedure, the lever's limited range of physical motion is seen in NAZA as the FULL RANGE of the gimbal. So, after you calibrate and click "Finish", moving the lever from side to side will move the gimbal through its full range of motion.
3) On my installation the "Center" setting on the Gimbal screen had no effect. But I was able to set the gimbal's center position by returning to the calibration screen, recalibrating, and then moving the cursor one unit (or as needed) in the required direction before clicking "Finish".
4) After calibration, the upper and lower limits of travel can be set by trial and error in the gimbal window without disturbing the calibration.
5) The few instruction videos I found for using the OEM FC40 camera on a gimbal mentioned the need to match the weight of the GoPro3 for which most gimbals are made. A GoPro3 weighs 78g, while the OEM camera weighs only 38g. Using Velcro, I attached a 9v battery (with the terminals snipped off and wrapped in electrical tape) to the right side of the bracket (facing the front of the FC40), with the camera flush with it on the left and hanging off slightly. At a little more than 40g, the battery was the correct weight and width to provide an almost perfect balance. The gimbal now works great!
The picture shows my Walkera G-2D installation. I used 8.5 mm thick slices of PVC plumbing pipe, attached with nylon straps, to improve ground clearance. They're very rigid and light in weight, and I did not have to fiddle with the compass sensor.